In a case that devices are produced by using a semiconductor single crystal wafer such as a silicon wafer, there are many steps from processing of the wafer to formation of a device. There is a step of heat treatment as one of these steps. The heat treatment step is an important process that is performed for the purpose of formation of a defect-free layer in a surface layer of the wafer, gettering, crystallization, formation of oxide film, impurity diffusion, and the like.
As a diffusion furnace used in such heat treatment step such as oxidization or impurity diffusion (an apparatus for oxidization and diffusion), along with a diameter of a wafer being larger, there has been mainly used a vertical furnace for heat treatment in which the heat treatment is performed with many wafers being horizontally supported at a predetermined interval. When a wafer is subjected to heat treatment by using such a vertical furnace for heat treatment, there is used a vertical boat for heat treatment (hereinafter, occasionally referred to as “a boat for heat treatment” or simply “a boat”) for setting many wafers.
FIG. 8 schematically shows a conventional general vertical boat for heat treatment 210. A pair of plate members (coupling members, also referred to as a top plate and a bottom plate) 216 are coupled to both ends of four supporting columns (rods) 214. Many slits 211 are formed in each of the supporting columns 214 and a convex part between the slits 211 acts as a supporting part 212 for the wafer. When wafers are subjected to heat treatment, the peripheral part of a wafer W is placed on the supporting parts 212 formed at the same height in each of the supporting columns 214, as shown in a plane view in FIG. 9(A) and a front view in FIG. 9(B), and thereby the wafer W is horizontally supported.
FIG. 10 is a schematic view for showing an example of a vertical furnace for heat treatment. In a boat for heat treatment 210 carried into an interior of a reaction chamber 222 of the vertical furnace for heat treatment 220, many wafers W are supported horizontally. At heat treatment, the wafers W are heated with a heater 224 provided around the reaction chamber 222. During heat treatment, gas is introduced into the reaction chamber 222 through a gas-introducing duct 226, flows from the upper side to the lower side, and is discharged outside from a gas-discharging duct 228. The gas to be used is different according to a purpose of heat treatment. However there are mainly used H2, N2, O2, Ar, and the like. In the case of impurity diffusion, these gasses are also used as a carrier gas for an impurity compound gas.
Various shapes are employed for a wafer supporting part 212 in a vertical boat for heat treatment 210, some examples of which are shown in FIGS. 11(A) and (B). In (A), semicircular supporting parts 212 are formed by providing concave slits (grooves) 211 in a cylindrical supporting column 214. In (B), on the other hand, rectangular supporting parts 213 are formed by providing concave slits 211 in a wide prismatic supporting column 215 so as to support wafers W at a position closer to the center than in the example shown in (A). There are other supporting parts having slits with other shapes such as arc shape or hook shape.
There are also proposed a relatively large plate-shaped supporting part (supporting plate) provided in a supporting column so as to support a wafer in a stable manner (see Japanese Unexamined Patent Publication (Kokai) No. 2000-53497), a supporting part having steps on its upper surface so as to support wafers having a different diameter (see Japanese Unexamined Patent Publication (Kokai) No. 2005-159028), and the like.
As for a material of a boat used for silicon wafers, for example, materials such as quartz (SiO2), silicon carbide (SiC), silicon (Si), and the like are usually used in order to prevent wafers from being contaminated. During heat treatment at a high temperature more than 1000° C., for example, a boat made from SiC or Si, which has higher heat resistance than a boat made from quartz (SiO2), is preferably used. Especially, a boat made from SiC is frequently used because it can be processed more easily, and it can more greatly reduce metallic contamination generated during heat treatment by providing CVD-SIC coating, compared to a boat made from Si.
When a heat treatment is performed at a high temperature especially for the purpose of oxidation or diffusion of impurities using a vertical boat for heat treatment, internal stress is caused due to a wafer's own weight, or thermal stress is caused due to non-uniform temperature distribution in the wafer. If such a stress exceeds a critical value, a slip dislocation, which is a crystal defect, is generated in the wafer. Since this critical value for the generation of this dislocation becomes rapidly small at a higher temperature, it is known that the slip dislocation is easily generated with a higher temperature. If a device is formed at a location where a slip dislocation has been generated, junction leakage and the like are caused, so that device production yield was sometimes remarkably reduced.
When a conventional boat with supporting parts 212 or 213 being formed is used as shown in FIG. 11(A) or (B), for example, slip dislocation is easily generated at a location of a wafer which comes in contact with each of the tips of the supporting parts 212 or 213. This is because point contact is sometimes caused at such a tip.
Especially in a case of a boat for heat treatment coated with CVD-SiC, its surface is very rough, i.e., has Ra (center line average roughness) of about 1 μm. When a wafer is placed on such a supporting part, the wafer is considered to be supported at a very small elevated portion (local projection) as the point contact. Thus the internal stress due to the wafer's own weight is considered to be increased locally, so that slip dislocation is easily generated.
In order to prevent generation of such slip dislocation, measures are taken such that the tip of the supporting part is chamfered or that the elevated portion of the wafer supporting part is removed by polishing its surface.
But a supporting part of a boat for heat treatment has a problem of being broken easily by a chamfering or polishing processing by means of a machine or the like because it is thin and fragile. If a single supporting part is broken, a whole boat becomes a failure product. Therefore, it becomes necessary to polish manually surfaces of the supporting parts in order to completely mirror-polish the surfaces. In that case, roughness in each surface of the supporting parts tends to vary. In addition, since mirror-polishing of all supporting part surfaces requires a lot of labor, so that the resulting boat becomes very expensive.
Besides, in order to establish the optimal shape of the supporting part with respect to surface roughness, a chamfer of the tip, and the like, various boats for heat treatment having various surface roughness and various chamfered shapes need to be made, and a lot of experiments need to be conducted in advance. As boats for heat treatment are expensive, preparing various boats for heat treatment used for the experiments is very expensive.
In order to solve these problems, Japanese Unexamined Patent Publication (Kokai) No. 2004-241545 discloses a boat having wafer supporting parts to which auxiliary supporting members are removably attached. According to the disclosure, in the case of such a boat, since the auxiliary supporting member can be removably attached, it can be subject to chamfering or polishing processing inexpensively and easily as desired on its surface for placing a wafer, and furthermore, when the polished or otherwise processed auxiliary supporting member is attached to the supporting part and then a wafer is placed on it to be subjected to heat treatment, generation of slip dislocation can be suppressed effectively.